Apparatuses for the airless spraying of paint and other fluids are well known. Airless paint sprayers are advantageous over compressed air sprayers because they are able to apply the paint at a significantly faster rate than air sprayers. They are particularly well suited for commercial, interior and exterior uses. They do not produce as fine a finish as air sprayers, so they are usable where a moderate finish is acceptable.
A typical airless paint spraying apparatus includes a fluid pump which delivers pressurized paint to a spray gun. The spray gun includes a nozzle particularly adapted for the corresponding fluid and pressures. Paint is supplied to the pump through an inlet tube, which may include a suction screen, that is placed directly into a paint can. The paint can typically used ranges in size from a five gallon can up to a fifty-five gallon can, depending upon the particular application. Paint is pumped from the can and through a flexible hose to the spray gun from which it is sprayed on the surface being painted.
The present invention is directed to a fluid paint pump assembly which includes a reciprocating pump. Many different sources of reciprocating motion to drive the pump are known in the art. One method is to use a hydraulic pump which powers a reciprocating hydraulic motor. Through a series of internal valves, the hydraulic motor reciprocates between two positions while the hydraulic fluid continuously flows in one direction. The reciprocating output of such a hydraulic motor is connected to a piston of the reciprocating fluid pump. The hydraulic pump may be driven by a wide range of rotational power sources, such as electric or gasoline engines.
The use of reciprocating hydraulic motors has some inherent disadvantages associated with it. First, there are the accompanying parasitic losses inherent in the hydraulic transfer of power and motion. Second, there is a practical limit to the cycle time of a reciprocating hydraulic motor. Additionally, there is an associated degree of complexity due to the need to supply a hydraulic fluid reservoir and filter.
As is also well known in the industry, a reciprocating fluid pump may be directly driven by a rotational power source through the use of a mechanism which translates rotational power and motion into reciprocating power and motion through the use of various mechanical mechanisms. However, many motion translators known in the prior art also translate some of the motion into side loading of the piston. The side forces can cause premature wear and failure of the seals or piston.
Depending on the specific rotational power source, a speed reducer may need to be located between the power source and the motion translator. One method of speed reduction is the use of gear sets and offset shafts. Another method is the use of planetary gear sets which include planet carriers which carry and locate planet gears between a ring gear and a sun gear. In either case, in order to obtain the necessary speed reduction, which may range as high 20:1 and beyond, such speed reducers can be expensive to manufacture and assemble, as well as bulky and noisy. All of these problems are objectional to the manufacturer as well as the end user.
The fluid pump is also well known in the industry. It consists of a reciprocating piston and packing glands which create a series of variable volume fluid chambers. As the piston is reciprocated between two positions, fluid is drawn into the pump through an intake siphon tube and past a one way check valve. When the piston reverses direction, the check valve closes and fluid is forced out the pump outlet through a hose to a spray gun. The piston itself typically carries a second check valve which provides the valving necessary to create the flow through the pump.
With a double acting pump, fluid is pumped out of the outlet, independently of the direction of travel of the piston. However, fluid is only drawn into the pump inlet during one-half of the cycle of the piston.
In such prior art pistons, a fluid passage way may be formed internally about the axis of reciprocation of the piston. At one end, the piston opens to a fluid pump chamber which directly communicates with the fluid inlet. Intermediate the ends of the piston, a hole is cross drilled, perpendicular to the axis of reciprocation of the piston, which communicates with the opposite end of the internal chamber of the piston and a second fluid pump chamber. Usually a plurality of such holes are so drilled. This forms outlet passages and exits whereby the fluid flows along the axis of reciprocation of the piston and exits the internal cavity at a right angle thereto. Because of the geometry of the pump, this flow through the internal cavity of the piston occurs during only one-half of the cycle of the piston.
Several problems are experienced with such prior art fluid pumps. First, as will be described later, the cross drilled outlet passages and exits decrease the efficiency of the fluid pump, in part due to the right angle change in direction induced in the flow of fluid. Such right angle outlet passages and exits also add length to the piston itself. Usually the piston, as well as the pump housing are formed of a stainless steel material. Such additional length adds to the cost of manufacture, as well as the cost of replacement parts for the pump.
Another problem with such prior art fluid pumps, is the inaccessibility of the packing glands which separate the fluid chambers from each other, as well as from the ambient environment. Efficient operation of such fluid pumps requires that optimum sealing be provided by the packing glands. Optimum sealing is a balance between adequate sealing force and excessive drag (resulting in wear, friction, heat generation and power consumption). Typical prior art pumps do not provide any means for externally individually compensating for aging and wearing of the individual packing gland, independent of the other packing gland.
Additionally, it is known in the art to have one of the packing glands carried by the piston. This results in one of the packing glands having relative motion with respect to one section of the piston, while the other packing gland has relative motion with respect to the housing. Such a design causes wear on both the piston and the housing. This necessitates the costly replacement of both the piston and the housing when the packing glands wear these components beyond their service limit.
As mentioned above, two one-way check valves are disposed within the fluid pump. These check valves provide the proper valving necessary to pump fluid from the inlet to the outlet. During each full cycle stroke of the piston, each check valve will be opened for approximately one-half of the cycle and closed for the remaining half of the cycle. Frequently these check valves are comprised of steel balls and valve seats. The ball comes into sealing engagement with its respective valve seat when the pressure downstream of the valve is greater than the pressure upstream of the valve. Paint is a relatively abrasive fluid, which causes wear on the valve seats. The cyclical seating and unseating of the ball also tends to wear the valve seat. In order to obtain acceptable life out of the valve seat, they may be formed of a very hard material, such as tungsten carbide. Because of the expense of such materials, the seats may be designed with two sealing surfaces. Prior art designs however, require that the valve seats be relatively large, which results in a greater total volumetric use of these expensive materials.
The prior art valve seats are typically designed with flat or squared off sealing surfaces. In order to allow fluid to flow through the check valve, the ball must be displaced a sufficient distance from the seat so as to allow adequate flow. This distance must also be sufficiently small so as to provide for a quick return of the ball to the seat to prevent reverse flow when the direction of reciprocation changes. The sharp edged seats result in turbulent flow between the seat and the check valve. This can result in the build up of paint deposits on the seat, thereby decreasing the sealing effect.